I highly recommend "Linear Systems and Signals" by B. P. Lathi . The author really talks to you as a student and does not introduce concepts without letting you know where he is going, and he does this in the most down to earth manner. Furthermore, he has great examples that are related to the material covered in the chapter. I also like the fact that matlab code is also introduced. IMHO, this book should be a keeper for anyone studying signals and systems. Lathi along with Oppenheim and you can't go wrong. Of course you should have the necessary mathematical background such as calc I-III, differential equations, and linear algebra. A year of calculus based physics would also be helpful but is not required.